Scientists believe shark deterrents may not be sufficiently effective

April 9, 2014 | Environment
Nurse Shark: nice sense of smell

Scientists have demonstrated that sharks are capable of attending to multiple sensory cues simultaneously.

Sharks use all of their senses when hunting for prey, according to a study led by researchers of the Mote Marine Laboratory, University of South Florida and Boston University.

Switching sensory modalities in a hierarchical fashion as they approach their prey, and substituting alternate sensory cues, when necessary, to accomplish behavioral tasks.

"This flexibility in behavior suggests that sharks are well adapted to succeed, even in the face of a changing environment and evolutionary advancements in prey defenses, including chemical, visual, and mechanical camouflage," the study tells.

"Closer to the prey, as more sensory cues became available, the preferred sensory modalities varied among species, with vision, hydrodynamic imaging, electroreception, and touch being important for orienting to, striking at, and capturing the prey."

Capture is most dependent on electroreception to coordinate the 10–100 millisecond-scale ram-suction movements. Touch can occasionally lead to capture when electroreception is blocked.

When researchers blocked both vision and lateral line, blacktip and bonnethead sharks could not follow the odor trail to locate prey, but nurse sharks could.

When the sharks' vision was blocked, removing a key sense for aiming at prey from long distances, they could compensate by lining up their strikes, albeit at closer range, using the lateral line, which can sense water movements from struggling prey.

With electroreception blocked, sharks usually failed to capture prey. However, blacktip and nurse sharks sometimes opened their mouths at the right time if their jaws touched prey, whereas touch did not help bonnetheads.

While the results do not focus on shark-and-human interactions, they do highlight that some shark-safety measures, like specially patterned wetsuits meant to provide visual camouflage or electrical deterrents that target the sharks' electrosensory system - each focusing on one sense at a time - may not be enough to change the rates of shark incidents.